Evacuated tubes for solar thermal energy collection typically contain an outer tube and an inner heat pipe. The outer tube has two walls, and a heat absorbing material coats the inside of the inner wall. The space between the walls is evacuated to prevent heat loss from the inner wall to the environment outside the tube. The inner heat pipe is hollow, and the space inside the pipe is substantially evacuated except for a small quantity of liquid, such as alcohol or water, that is contained inside the inner heat pipe.
Sunlight shining on the tube passes through the transparent outer wall and heats the heat absorbing material on the inner wall of the tube. The heat absorbed by the heat absorbing material is not lost to the environment, and instead heats the heat pipe. This causes the liquid in the heat pipe to vaporize and rise to the top of the pipe. A heat transfer fluid, such as water or glycol, flows through a header tube around the upper ends of the heat pipes and absorbs heat from the heat pipe vapor. The loss of heat from the vapor to the heat transfer fluid causes the vapor in the heat pipe to condense and flows back down into heating portion of the heat pipe. As the process repeats, solar energy continues to be absorbed by the heat pipes, used to vaporize the heat pipe fluid, and transferred from the heat pipes to the heat transfer fluid in the header as the heat pipe vapor condenses. The efficiency with which heat is transferred from the heat pipes to the header fluid is one determinant of the efficiency of the device.
A need exists for solar thermal energy collectors having improved heat transfer between the heat pipes and the header fluid. The present invention addresses that need.